In Follow me,Why sip extension ring so long time?

There are serveral sip extensions(eg. 7000,7001,7002) in follow me of 7000. Strategy is hunt, confirm checked.i call 7000,extension 7000 ring, i didn't pickup it. then 7001 ring,but at the same time 7000 still ring. this is 7000 and 7001 all ring at the same time,serveral seconds past ,7000 stop ring.my question is that this may cause somebody pick 7000,but when you pick it, be promoted line is not available,in fact you should pick 7001. how to Correct it?